Four bombs that each contained less than 10 pounds of explosives and were small enough to be concealed and carried in backpacks caused the carnage in London's mass transit system that killed at least 49 people and injured 700, police officials announced Friday.

More than one person planted the explosives, police said, and there was no evidence so far that the attacks -- the worst in London since World War II -- were suicide bombings. Ian Blair, head of the city's police force, warned that the perpetrators were probably still at large, and he urged public vigilance.

The official count of 49 did not include an unknown number of bodies still trapped in the wreckage of a subway car wedged 70 feet below ground in a tunnel near the Russell Square subway station. That attack caused the largest toll of death and devastation in the Thursday morning rush-hour bombings.

Senior officials confirmed that they had downgraded the security alert level in May and had no warning of the attacks. Charles Clarke, the cabinet secretary in charge of domestic security, defended Britain's intelligence services but conceded that there had been "a failure of intelligence in the sense that we didn't know this was coming."

As wary Londoners returned to their offices and shops Friday morning, emergency workers and investigators pored over charred and twisted wreckage at four bomb sites, searching for victims and clues to the identity of the terrorists.

Officials said no arrests had been made, and they publicly identified no suspects. But they said it was likely the perpetrators were associated with the al Qaeda network. "The number one task today is to track these people down," Clarke told reporters.

One working theory, said an official with knowledge of the investigation, was that the bombers had originated from King's Cross station and fanned out to various subway lines and the No. 30 double-decker bus, which left from nearby Euston station. Officials said it was still unclear whether mechanical timers or cell phones had been used to set off the blasts.

Investigators were gathering tapes from dozens of security cameras located in the stations and the subway cars. More than 104,000 people had phoned the anti-terror hotline. "We've got lots and lots of witnesses," Blair said.

London Mayor Ken Livingstone and other leaders expressed pride and defiance and pleaded with Londoners to return to normal life. Family members and friends of the missing searched hospitals and distributed makeshift posters of their loved ones.

After a preliminary examination of the crime scenes, investigators said the bombs had been placed on the floors of three crowded train cars and detonated within minutes of each other, beginning at 8:51 a.m. The fourth bomb ripped through the bus almost an hour later, spattering blood, body parts and debris over a large swath of historic Tavistock Square.

The explosion on the bus was so intense that officials said it was Friday afternoon before they were able to retrieve the bodies and produce an accurate death count, 13. They would not comment on reports from witnesses that a man in an agitated state had been on board moments before the blast.

"He was standing next to me with a bag at his feet, and he kept dipping into this bag and fiddling about with something," Richard Jones, an information technology consultant, told the BBC. Jones, who gave a statement to police, said he left the bus because it was moving too slowly.

Investigators said the wreckage of the bus could prove a rich environment for clues because it is above ground and accessible and because one of the dead might have been the bomber. Some analysts said the bomber could have been thwarted from entering nearby Euston station, which was shut down after the first blasts, and boarded the bus in order to blow it up or ride to another potential target.

"Perhaps in panicking he jumped on a bus and tried to turn off the device," said Dominic Armstrong, director of research and intelligence for Aegis Defense Services, a London-based private security firm. "It's too anomalous to have been a concerted attack on London buses."

"The next phase is very dangerous," he added, "because the security forces now have to track these people down. Their manual says you don't submit to capture, you kill yourself and take as many of the enemy with you as you can."

The Piccadilly Line train car located deep in the bowels of the London Underground about 500 yards from the Russell Square station posed a far more difficult and dangerous problem for rescuers and investigators. Twenty-one bodies have been removed from the site, but an indeterminate number are still trapped inside.

The bomb was placed in the front car near a set of double doors, police said, and the blast damaged the tunnel as well as shattering the car. They said that rescuers had been able to enter the car and determine no survivors remained but that it was too dangerous for emergency workers to remove the bodies. "It is a very, very large job, and a job of extraordinary horror," Blair said.

Asked whether the perpetrators were still a threat, Blair replied: "It's an answer which is blindingly obvious -- there is likely to still be a cell. Whether these people are still in the United Kingdom is a question. We must remain vigilant."

The vast underground crime scenes posed a particularly thorny problem for investigators, but after 30 years of dealing with the threat from Irish Republican Army bombers, the authorities said they were well equipped to handle the forensic and intelligence work.

Still, they pleaded with the public for help. Any signs of suspicious activity -- including reports of tenants who had suddenly abandoned rental properties or storage sheds -- could be vital. "It is not the police and it is not the intelligence services who will defeat terrorism; it is communities who defeat terrorism," Blair said.

Security officials had long feared and predicted an attack on London's subway system similar to the Madrid train bombings that killed 191 people in March 2004, and they had recently staged a training exercise involving a simulated attack on multiple train lines.

But fears receded after there was no strike before the May 5 general election, and the government's Joint Terrorist Analysis Center lowered the threat level from "severe general" to "substantial" later that month. Just 90 minutes before Thursday's first bomb struck, Scotland Yard's Blair had told a BBC radio interviewer his forces were "the envy of the policing world in relation to counterterrorism."

Officials leapt to the defense of the police and intelligence services. Home Secretary Clarke said the change in threat level had not affected the vigilance of security forces. "We have a very comprehensive intelligence service," Clarke told the BBC in one of a series of interviews he did Friday.

Riders on a double-decker bus look out at the site of a blast that tore apart a similar bus Thursday. Officials say small bombs were used in the attacks and that more than one person planned them.Queen Elizabeth II visits with Bruce Lait at a London hospital. Lait was on a subway train Thursday. The attacks left 700 people injured. Officials said it was likely the perpetrators were associated with the al Qaeda network. London resident Yvonne Nash, center, whose boyfriend, Jamie Gordon, is missing, reacts as she stands near the scene of one of the bus explosions.